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cpbtrf.f
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1 *> \brief \b CPBTRF
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
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16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE CPBTRF( UPLO, N, KD, AB, LDAB, INFO )
22 *
23 * .. Scalar Arguments ..
24 * CHARACTER UPLO
25 * INTEGER INFO, KD, LDAB, N
26 * ..
27 * .. Array Arguments ..
28 * COMPLEX AB( LDAB, * )
29 * ..
30 *
31 *
32 *> \par Purpose:
33 * =============
34 *>
35 *> \verbatim
36 *>
37 *> CPBTRF computes the Cholesky factorization of a complex Hermitian
38 *> positive definite band matrix A.
39 *>
40 *> The factorization has the form
41 *> A = U**H * U, if UPLO = 'U', or
42 *> A = L * L**H, if UPLO = 'L',
43 *> where U is an upper triangular matrix and L is lower triangular.
44 *> \endverbatim
45 *
46 * Arguments:
47 * ==========
48 *
49 *> \param[in] UPLO
50 *> \verbatim
51 *> UPLO is CHARACTER*1
52 *> = 'U': Upper triangle of A is stored;
53 *> = 'L': Lower triangle of A is stored.
54 *> \endverbatim
55 *>
56 *> \param[in] N
57 *> \verbatim
58 *> N is INTEGER
59 *> The order of the matrix A. N >= 0.
60 *> \endverbatim
61 *>
62 *> \param[in] KD
63 *> \verbatim
64 *> KD is INTEGER
65 *> The number of superdiagonals of the matrix A if UPLO = 'U',
66 *> or the number of subdiagonals if UPLO = 'L'. KD >= 0.
67 *> \endverbatim
68 *>
69 *> \param[in,out] AB
70 *> \verbatim
71 *> AB is COMPLEX array, dimension (LDAB,N)
72 *> On entry, the upper or lower triangle of the Hermitian band
73 *> matrix A, stored in the first KD+1 rows of the array. The
74 *> j-th column of A is stored in the j-th column of the array AB
75 *> as follows:
76 *> if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
77 *> if UPLO = 'L', AB(1+i-j,j) = A(i,j) for j<=i<=min(n,j+kd).
78 *>
79 *> On exit, if INFO = 0, the triangular factor U or L from the
80 *> Cholesky factorization A = U**H*U or A = L*L**H of the band
81 *> matrix A, in the same storage format as A.
82 *> \endverbatim
83 *>
84 *> \param[in] LDAB
85 *> \verbatim
86 *> LDAB is INTEGER
87 *> The leading dimension of the array AB. LDAB >= KD+1.
88 *> \endverbatim
89 *>
90 *> \param[out] INFO
91 *> \verbatim
92 *> INFO is INTEGER
93 *> = 0: successful exit
94 *> < 0: if INFO = -i, the i-th argument had an illegal value
95 *> > 0: if INFO = i, the leading minor of order i is not
96 *> positive definite, and the factorization could not be
97 *> completed.
98 *> \endverbatim
99 *
100 * Authors:
101 * ========
102 *
103 *> \author Univ. of Tennessee
104 *> \author Univ. of California Berkeley
105 *> \author Univ. of Colorado Denver
106 *> \author NAG Ltd.
107 *
108 *> \date November 2011
109 *
110 *> \ingroup complexOTHERcomputational
111 *
112 *> \par Further Details:
113 * =====================
114 *>
115 *> \verbatim
116 *>
117 *> The band storage scheme is illustrated by the following example, when
118 *> N = 6, KD = 2, and UPLO = 'U':
119 *>
120 *> On entry: On exit:
121 *>
122 *> * * a13 a24 a35 a46 * * u13 u24 u35 u46
123 *> * a12 a23 a34 a45 a56 * u12 u23 u34 u45 u56
124 *> a11 a22 a33 a44 a55 a66 u11 u22 u33 u44 u55 u66
125 *>
126 *> Similarly, if UPLO = 'L' the format of A is as follows:
127 *>
128 *> On entry: On exit:
129 *>
130 *> a11 a22 a33 a44 a55 a66 l11 l22 l33 l44 l55 l66
131 *> a21 a32 a43 a54 a65 * l21 l32 l43 l54 l65 *
132 *> a31 a42 a53 a64 * * l31 l42 l53 l64 * *
133 *>
134 *> Array elements marked * are not used by the routine.
135 *> \endverbatim
136 *
137 *> \par Contributors:
138 * ==================
139 *>
140 *> Peter Mayes and Giuseppe Radicati, IBM ECSEC, Rome, March 23, 1989
141 *
142 * =====================================================================
143  SUBROUTINE cpbtrf( UPLO, N, KD, AB, LDAB, INFO )
144 *
145 * -- LAPACK computational routine (version 3.4.0) --
146 * -- LAPACK is a software package provided by Univ. of Tennessee, --
147 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
148 * November 2011
149 *
150 * .. Scalar Arguments ..
151  CHARACTER uplo
152  INTEGER info, kd, ldab, n
153 * ..
154 * .. Array Arguments ..
155  COMPLEX ab( ldab, * )
156 * ..
157 *
158 * =====================================================================
159 *
160 * .. Parameters ..
161  REAL one, zero
162  parameter( one = 1.0e+0, zero = 0.0e+0 )
163  COMPLEX cone
164  parameter( cone = ( 1.0e+0, 0.0e+0 ) )
165  INTEGER nbmax, ldwork
166  parameter( nbmax = 32, ldwork = nbmax+1 )
167 * ..
168 * .. Local Scalars ..
169  INTEGER i, i2, i3, ib, ii, j, jj, nb
170 * ..
171 * .. Local Arrays ..
172  COMPLEX work( ldwork, nbmax )
173 * ..
174 * .. External Functions ..
175  LOGICAL lsame
176  INTEGER ilaenv
177  EXTERNAL lsame, ilaenv
178 * ..
179 * .. External Subroutines ..
180  EXTERNAL cgemm, cherk, cpbtf2, cpotf2, ctrsm, xerbla
181 * ..
182 * .. Intrinsic Functions ..
183  INTRINSIC min
184 * ..
185 * .. Executable Statements ..
186 *
187 * Test the input parameters.
188 *
189  info = 0
190  IF( ( .NOT.lsame( uplo, 'U' ) ) .AND.
191  $ ( .NOT.lsame( uplo, 'L' ) ) ) THEN
192  info = -1
193  ELSE IF( n.LT.0 ) THEN
194  info = -2
195  ELSE IF( kd.LT.0 ) THEN
196  info = -3
197  ELSE IF( ldab.LT.kd+1 ) THEN
198  info = -5
199  END IF
200  IF( info.NE.0 ) THEN
201  CALL xerbla( 'CPBTRF', -info )
202  return
203  END IF
204 *
205 * Quick return if possible
206 *
207  IF( n.EQ.0 )
208  $ return
209 *
210 * Determine the block size for this environment
211 *
212  nb = ilaenv( 1, 'CPBTRF', uplo, n, kd, -1, -1 )
213 *
214 * The block size must not exceed the semi-bandwidth KD, and must not
215 * exceed the limit set by the size of the local array WORK.
216 *
217  nb = min( nb, nbmax )
218 *
219  IF( nb.LE.1 .OR. nb.GT.kd ) THEN
220 *
221 * Use unblocked code
222 *
223  CALL cpbtf2( uplo, n, kd, ab, ldab, info )
224  ELSE
225 *
226 * Use blocked code
227 *
228  IF( lsame( uplo, 'U' ) ) THEN
229 *
230 * Compute the Cholesky factorization of a Hermitian band
231 * matrix, given the upper triangle of the matrix in band
232 * storage.
233 *
234 * Zero the upper triangle of the work array.
235 *
236  DO 20 j = 1, nb
237  DO 10 i = 1, j - 1
238  work( i, j ) = zero
239  10 continue
240  20 continue
241 *
242 * Process the band matrix one diagonal block at a time.
243 *
244  DO 70 i = 1, n, nb
245  ib = min( nb, n-i+1 )
246 *
247 * Factorize the diagonal block
248 *
249  CALL cpotf2( uplo, ib, ab( kd+1, i ), ldab-1, ii )
250  IF( ii.NE.0 ) THEN
251  info = i + ii - 1
252  go to 150
253  END IF
254  IF( i+ib.LE.n ) THEN
255 *
256 * Update the relevant part of the trailing submatrix.
257 * If A11 denotes the diagonal block which has just been
258 * factorized, then we need to update the remaining
259 * blocks in the diagram:
260 *
261 * A11 A12 A13
262 * A22 A23
263 * A33
264 *
265 * The numbers of rows and columns in the partitioning
266 * are IB, I2, I3 respectively. The blocks A12, A22 and
267 * A23 are empty if IB = KD. The upper triangle of A13
268 * lies outside the band.
269 *
270  i2 = min( kd-ib, n-i-ib+1 )
271  i3 = min( ib, n-i-kd+1 )
272 *
273  IF( i2.GT.0 ) THEN
274 *
275 * Update A12
276 *
277  CALL ctrsm( 'Left', 'Upper', 'Conjugate transpose',
278  $ 'Non-unit', ib, i2, cone,
279  $ ab( kd+1, i ), ldab-1,
280  $ ab( kd+1-ib, i+ib ), ldab-1 )
281 *
282 * Update A22
283 *
284  CALL cherk( 'Upper', 'Conjugate transpose', i2, ib,
285  $ -one, ab( kd+1-ib, i+ib ), ldab-1, one,
286  $ ab( kd+1, i+ib ), ldab-1 )
287  END IF
288 *
289  IF( i3.GT.0 ) THEN
290 *
291 * Copy the lower triangle of A13 into the work array.
292 *
293  DO 40 jj = 1, i3
294  DO 30 ii = jj, ib
295  work( ii, jj ) = ab( ii-jj+1, jj+i+kd-1 )
296  30 continue
297  40 continue
298 *
299 * Update A13 (in the work array).
300 *
301  CALL ctrsm( 'Left', 'Upper', 'Conjugate transpose',
302  $ 'Non-unit', ib, i3, cone,
303  $ ab( kd+1, i ), ldab-1, work, ldwork )
304 *
305 * Update A23
306 *
307  IF( i2.GT.0 )
308  $ CALL cgemm( 'Conjugate transpose',
309  $ 'No transpose', i2, i3, ib, -cone,
310  $ ab( kd+1-ib, i+ib ), ldab-1, work,
311  $ ldwork, cone, ab( 1+ib, i+kd ),
312  $ ldab-1 )
313 *
314 * Update A33
315 *
316  CALL cherk( 'Upper', 'Conjugate transpose', i3, ib,
317  $ -one, work, ldwork, one,
318  $ ab( kd+1, i+kd ), ldab-1 )
319 *
320 * Copy the lower triangle of A13 back into place.
321 *
322  DO 60 jj = 1, i3
323  DO 50 ii = jj, ib
324  ab( ii-jj+1, jj+i+kd-1 ) = work( ii, jj )
325  50 continue
326  60 continue
327  END IF
328  END IF
329  70 continue
330  ELSE
331 *
332 * Compute the Cholesky factorization of a Hermitian band
333 * matrix, given the lower triangle of the matrix in band
334 * storage.
335 *
336 * Zero the lower triangle of the work array.
337 *
338  DO 90 j = 1, nb
339  DO 80 i = j + 1, nb
340  work( i, j ) = zero
341  80 continue
342  90 continue
343 *
344 * Process the band matrix one diagonal block at a time.
345 *
346  DO 140 i = 1, n, nb
347  ib = min( nb, n-i+1 )
348 *
349 * Factorize the diagonal block
350 *
351  CALL cpotf2( uplo, ib, ab( 1, i ), ldab-1, ii )
352  IF( ii.NE.0 ) THEN
353  info = i + ii - 1
354  go to 150
355  END IF
356  IF( i+ib.LE.n ) THEN
357 *
358 * Update the relevant part of the trailing submatrix.
359 * If A11 denotes the diagonal block which has just been
360 * factorized, then we need to update the remaining
361 * blocks in the diagram:
362 *
363 * A11
364 * A21 A22
365 * A31 A32 A33
366 *
367 * The numbers of rows and columns in the partitioning
368 * are IB, I2, I3 respectively. The blocks A21, A22 and
369 * A32 are empty if IB = KD. The lower triangle of A31
370 * lies outside the band.
371 *
372  i2 = min( kd-ib, n-i-ib+1 )
373  i3 = min( ib, n-i-kd+1 )
374 *
375  IF( i2.GT.0 ) THEN
376 *
377 * Update A21
378 *
379  CALL ctrsm( 'Right', 'Lower',
380  $ 'Conjugate transpose', 'Non-unit', i2,
381  $ ib, cone, ab( 1, i ), ldab-1,
382  $ ab( 1+ib, i ), ldab-1 )
383 *
384 * Update A22
385 *
386  CALL cherk( 'Lower', 'No transpose', i2, ib, -one,
387  $ ab( 1+ib, i ), ldab-1, one,
388  $ ab( 1, i+ib ), ldab-1 )
389  END IF
390 *
391  IF( i3.GT.0 ) THEN
392 *
393 * Copy the upper triangle of A31 into the work array.
394 *
395  DO 110 jj = 1, ib
396  DO 100 ii = 1, min( jj, i3 )
397  work( ii, jj ) = ab( kd+1-jj+ii, jj+i-1 )
398  100 continue
399  110 continue
400 *
401 * Update A31 (in the work array).
402 *
403  CALL ctrsm( 'Right', 'Lower',
404  $ 'Conjugate transpose', 'Non-unit', i3,
405  $ ib, cone, ab( 1, i ), ldab-1, work,
406  $ ldwork )
407 *
408 * Update A32
409 *
410  IF( i2.GT.0 )
411  $ CALL cgemm( 'No transpose',
412  $ 'Conjugate transpose', i3, i2, ib,
413  $ -cone, work, ldwork, ab( 1+ib, i ),
414  $ ldab-1, cone, ab( 1+kd-ib, i+ib ),
415  $ ldab-1 )
416 *
417 * Update A33
418 *
419  CALL cherk( 'Lower', 'No transpose', i3, ib, -one,
420  $ work, ldwork, one, ab( 1, i+kd ),
421  $ ldab-1 )
422 *
423 * Copy the upper triangle of A31 back into place.
424 *
425  DO 130 jj = 1, ib
426  DO 120 ii = 1, min( jj, i3 )
427  ab( kd+1-jj+ii, jj+i-1 ) = work( ii, jj )
428  120 continue
429  130 continue
430  END IF
431  END IF
432  140 continue
433  END IF
434  END IF
435  return
436 *
437  150 continue
438  return
439 *
440 * End of CPBTRF
441 *
442  END